The Same Bacteria That Rot Your Food Keep the Soil Fertile
See how bacteria give us antibiotics, serums and vaccines, follow nitrogen through the four kinds of soil bacteria, learn why tea and leather need them, and list the harm they do.
How can the same organism be a farmer's best friend and a cook's worst enemy?
Leave cooked rice out on a warm afternoon and by evening it smells sour and slimy. Bacteria have broken it down, and it is fit for nothing.
Now bury a heap of leaves and crop waste in a corner of the same field. Bacteria break that down too — and what is left is rich, dark, fertile soil that will grow next season's crop.
Same organisms. Same process. Decomposition is a disaster in a kitchen and a necessity in a field, and the bacteria have no idea which is which.
That is the honest shape of this whole chapter. Bacteria are not good or bad; they break organic matter down and they build nitrogen compounds, and whether we call the result useful depends entirely on where it happens and whether we wanted it.
On the useful side they give us antibiotics, serums and vaccines; they keep nitrogen circulating so that plants can grow at all; and they do jobs in industry that no machine does as cheaply — curing tea, tanning leather, setting curd. On the harmful side they spoil food, attack crops and cause some of the most serious diseases there are.
This page covers the first part of the ICSE Class 9 Biology chapter on economic importance: bacteria in medicine, their role in the nitrogen cycle and in agriculture, their industrial uses, and the harm they do.
Now bury a heap of leaves and crop waste in a corner of the same field. Bacteria break that down too — and what is left is rich, dark, fertile soil that will grow next season's crop.
Same organisms. Same process. Decomposition is a disaster in a kitchen and a necessity in a field, and the bacteria have no idea which is which.
That is the honest shape of this whole chapter. Bacteria are not good or bad; they break organic matter down and they build nitrogen compounds, and whether we call the result useful depends entirely on where it happens and whether we wanted it.
On the useful side they give us antibiotics, serums and vaccines; they keep nitrogen circulating so that plants can grow at all; and they do jobs in industry that no machine does as cheaply — curing tea, tanning leather, setting curd. On the harmful side they spoil food, attack crops and cause some of the most serious diseases there are.
This page covers the first part of the ICSE Class 9 Biology chapter on economic importance: bacteria in medicine, their role in the nitrogen cycle and in agriculture, their industrial uses, and the harm they do.
What do bacteria give us in medicine?
Bacteria supply antibiotics directly, and they are the raw material from which serums and vaccines are prepared.
Antibiotics. An antibiotic is a substance produced by a micro-organism which kills or stops the growth of other micro-organisms, and which can be taken into the body safely.
Antibiotics obtained from bacteria include:
- Streptomycin, from Streptomyces — used against tuberculosis
- Tetracycline
- Chloramphenicol
- Erythromycin
- Neomycin
- Bacitracin
They work by attacking something the bacterium needs and we do not — many prevent it from building its cell wall, others stop it making protein or copying its genetic material, so it cannot multiply and the body's own defences finish it off.
Serums. A serum contains ready-made antibodies. An animal, usually a horse, is injected with small increasing doses of a toxin or a killed pathogen until its blood is rich in the corresponding antibody. The blood is then drawn and the clear serum separated.
Injected into a patient, those borrowed antibodies neutralise the toxin at once. So a serum is used for treatment after exposure — anti-tetanus serum after a deep dirty wound, anti-diphtheria serum, antivenom for snake bite. The protection is immediate but short-lived, because the patient's own body never learnt to make the antibody.
Vaccines. A vaccine contains killed pathogens, weakened (attenuated) living ones, or their toxoids — inactivated toxins. It is harmless, but the body cannot tell the difference, so it is stimulated into producing its own antibodies and keeping memory of how to do so.
So a vaccine is used for prevention before exposure — BCG against tuberculosis, TAB against typhoid, DPT, cholera vaccine. The protection is slower to develop but long-lasting.
Keep the two apart, because this is the distinction most often mixed up. A serum hands the body a set of finished antibodies; a vaccine teaches the body to manufacture its own.
So a serum is a loan and a vaccine is a lesson. The loan arrives instantly and is soon spent; the lesson takes weeks to learn and is never forgotten. A question asking why a vaccine cannot be used to treat a person already bitten by a rabid animal is answered by exactly that — there is no time for the body to learn, so the serum is what is needed.
One boundary point worth having. Not every antibiotic comes from a bacterium. Penicillin is obtained from a fungus, Penicillium, which is why it belongs in the next part of this chapter. An answer listing penicillin among the bacterial antibiotics loses the mark, and it is a favourite one-line trap.
Antibiotics. An antibiotic is a substance produced by a micro-organism which kills or stops the growth of other micro-organisms, and which can be taken into the body safely.
Antibiotics obtained from bacteria include:
- Streptomycin, from Streptomyces — used against tuberculosis
- Tetracycline
- Chloramphenicol
- Erythromycin
- Neomycin
- Bacitracin
They work by attacking something the bacterium needs and we do not — many prevent it from building its cell wall, others stop it making protein or copying its genetic material, so it cannot multiply and the body's own defences finish it off.
Serums. A serum contains ready-made antibodies. An animal, usually a horse, is injected with small increasing doses of a toxin or a killed pathogen until its blood is rich in the corresponding antibody. The blood is then drawn and the clear serum separated.
Injected into a patient, those borrowed antibodies neutralise the toxin at once. So a serum is used for treatment after exposure — anti-tetanus serum after a deep dirty wound, anti-diphtheria serum, antivenom for snake bite. The protection is immediate but short-lived, because the patient's own body never learnt to make the antibody.
Vaccines. A vaccine contains killed pathogens, weakened (attenuated) living ones, or their toxoids — inactivated toxins. It is harmless, but the body cannot tell the difference, so it is stimulated into producing its own antibodies and keeping memory of how to do so.
So a vaccine is used for prevention before exposure — BCG against tuberculosis, TAB against typhoid, DPT, cholera vaccine. The protection is slower to develop but long-lasting.
Keep the two apart, because this is the distinction most often mixed up. A serum hands the body a set of finished antibodies; a vaccine teaches the body to manufacture its own.
So a serum is a loan and a vaccine is a lesson. The loan arrives instantly and is soon spent; the lesson takes weeks to learn and is never forgotten. A question asking why a vaccine cannot be used to treat a person already bitten by a rabid animal is answered by exactly that — there is no time for the body to learn, so the serum is what is needed.
One boundary point worth having. Not every antibiotic comes from a bacterium. Penicillin is obtained from a fungus, Penicillium, which is why it belongs in the next part of this chapter. An answer listing penicillin among the bacterial antibiotics loses the mark, and it is a favourite one-line trap.
How do four kinds of bacteria keep nitrogen moving?
Nitrogen makes up about four-fifths of the air, and no plant can use a single molecule of it directly. Bacteria are what convert it into a usable form and what return it to the air.
A plant needs nitrogen to make proteins, and it can absorb it only as nitrates through its roots. Getting from nitrogen gas to nitrate takes several steps, and a different group of bacteria handles each one.
Nitrogen-fixing bacteria — they convert atmospheric nitrogen into nitrogenous compounds. There are two kinds.
- Symbiotic — Rhizobium, which lives inside root nodules on the roots of leguminous plants. The plant gives the bacteria food and shelter; the bacteria give the plant fixed nitrogen. Legumes include pea, gram, groundnut, beans, soya and lucerne
- Free-living in the soil — Azotobacter and Clostridium. Also the blue-green algae Nostoc and Anabaena, which fix nitrogen in flooded paddy fields
Ammonifying (putrefying) bacteria — they act on the nitrogen in dead plants and animals and in excreta, converting it into ammonia. This is the step that recovers nitrogen already in use.
Nitrifying bacteria — they turn ammonia into the nitrate a root can absorb, in two stages carried out by two different genera.
- Nitrosomonas converts ammonia into nitrite
- Nitrobacter converts nitrite into nitrate
Denitrifying bacteria — Pseudomonas and Thiobacillus. They break nitrates down and release free nitrogen back into the air, completing the cycle.
Notice that the last group works against the farmer. Denitrifying bacteria remove from the soil exactly the nitrate the nitrifying bacteria worked to supply, so they lower soil fertility. They flourish in waterlogged, poorly aerated soil.
So they are harmful to agriculture and essential to the cycle at the same time. Without them the atmosphere's nitrogen would be steadily locked into the soil and never returned. That is why ploughing and draining a field is useful — it lets air into the soil and discourages the denitrifiers, without anybody having to remove them.
Why this matters in a field, in one practical rule. A crop of wheat or rice takes nitrogen out of the soil. A crop of gram, groundnut or beans puts it back, because Rhizobium in its root nodules is fixing nitrogen the whole time it grows.
So a farmer who alternates the two — crop rotation — restores the soil's nitrogen without buying a single bag of fertiliser. Ploughing the legume's remains back in adds more still, and this is why pulses and cereals have been grown in alternation in Indian fields for a very long time.
The bacteria are doing free of charge what a fertiliser factory does at great expense, and that is the most useful single sentence in this topic.
A plant needs nitrogen to make proteins, and it can absorb it only as nitrates through its roots. Getting from nitrogen gas to nitrate takes several steps, and a different group of bacteria handles each one.
Nitrogen-fixing bacteria — they convert atmospheric nitrogen into nitrogenous compounds. There are two kinds.
- Symbiotic — Rhizobium, which lives inside root nodules on the roots of leguminous plants. The plant gives the bacteria food and shelter; the bacteria give the plant fixed nitrogen. Legumes include pea, gram, groundnut, beans, soya and lucerne
- Free-living in the soil — Azotobacter and Clostridium. Also the blue-green algae Nostoc and Anabaena, which fix nitrogen in flooded paddy fields
Ammonifying (putrefying) bacteria — they act on the nitrogen in dead plants and animals and in excreta, converting it into ammonia. This is the step that recovers nitrogen already in use.
Nitrifying bacteria — they turn ammonia into the nitrate a root can absorb, in two stages carried out by two different genera.
- Nitrosomonas converts ammonia into nitrite
- Nitrobacter converts nitrite into nitrate
Denitrifying bacteria — Pseudomonas and Thiobacillus. They break nitrates down and release free nitrogen back into the air, completing the cycle.
Notice that the last group works against the farmer. Denitrifying bacteria remove from the soil exactly the nitrate the nitrifying bacteria worked to supply, so they lower soil fertility. They flourish in waterlogged, poorly aerated soil.
So they are harmful to agriculture and essential to the cycle at the same time. Without them the atmosphere's nitrogen would be steadily locked into the soil and never returned. That is why ploughing and draining a field is useful — it lets air into the soil and discourages the denitrifiers, without anybody having to remove them.
Why this matters in a field, in one practical rule. A crop of wheat or rice takes nitrogen out of the soil. A crop of gram, groundnut or beans puts it back, because Rhizobium in its root nodules is fixing nitrogen the whole time it grows.
So a farmer who alternates the two — crop rotation — restores the soil's nitrogen without buying a single bag of fertiliser. Ploughing the legume's remains back in adds more still, and this is why pulses and cereals have been grown in alternation in Indian fields for a very long time.
The bacteria are doing free of charge what a fertiliser factory does at great expense, and that is the most useful single sentence in this topic.
Why do tea leaves and animal hides need bacteria?
In both cases the bacteria perform a partial, controlled breakdown that no simple machine can do — developing flavour in one and loosening unwanted tissue in the other.
Curing of tea. Freshly plucked tea leaves have almost none of the taste or smell of tea. They are withered, rolled to break the cells open, and then spread in a warm, moist place and left.
During that period bacteria and the leaf's own enzymes act on the substances released, and the characteristic flavour, aroma and dark colour of tea develop. The leaves are then dried to stop the process at the right point.
The same kind of controlled bacterial action is used in curing tobacco, coffee and cocoa.
Tanning of leather. A raw hide taken from an animal carries hair on the outside and flesh and fat on the inside, and it would rot if left as it is.
The hide is soaked in water, and bacteria loosen the hair and the fat from the skin so that both can be scraped away cleanly without damaging the hide itself. This soaking stage is called curing. The cleaned hide is then treated with tannin or with chromium salts, which makes it durable, flexible and resistant to decay — the process that gives leather its name.
Other industrial uses worth a line each.
- Retting of fibres — the stems of jute, flax and hemp are steeped in water, and bacteria rot away the soft tissue so that the tough fibres can be separated for making rope, sacking and cloth
- Vinegar — Acetobacter converts alcohol into acetic acid
- Dairy products — Lactobacillus converts the lactose of milk into lactic acid, which curdles the milk protein; this gives curd, and butter and cheese follow from it
- Silage — green fodder is packed into a pit, and bacterial fermentation preserves it as cattle feed for the dry season
The curd in every Indian kitchen is this chemistry in action. A spoonful of old curd added to warm milk introduces Lactobacillus; within hours the lactose has become lactic acid, the acid has set the protein, and the milk is curd — which is also why curd tastes sour and why it will not set if the milk is too cold for the bacteria to work.
Now notice what these processes have in common, because it is the point of the section. In each one the bacteria are allowed to break something down partly and then stopped — by drying the tea, by scraping the hide, by refrigerating the curd.
A controlled, incomplete decay is a useful process; the same decay allowed to run to completion is spoilage. So the whole industrial skill lies in knowing when to stop, and that is the honest connection between this section and the harmful effects in the next one.
Curing of tea. Freshly plucked tea leaves have almost none of the taste or smell of tea. They are withered, rolled to break the cells open, and then spread in a warm, moist place and left.
During that period bacteria and the leaf's own enzymes act on the substances released, and the characteristic flavour, aroma and dark colour of tea develop. The leaves are then dried to stop the process at the right point.
The same kind of controlled bacterial action is used in curing tobacco, coffee and cocoa.
Tanning of leather. A raw hide taken from an animal carries hair on the outside and flesh and fat on the inside, and it would rot if left as it is.
The hide is soaked in water, and bacteria loosen the hair and the fat from the skin so that both can be scraped away cleanly without damaging the hide itself. This soaking stage is called curing. The cleaned hide is then treated with tannin or with chromium salts, which makes it durable, flexible and resistant to decay — the process that gives leather its name.
Other industrial uses worth a line each.
- Retting of fibres — the stems of jute, flax and hemp are steeped in water, and bacteria rot away the soft tissue so that the tough fibres can be separated for making rope, sacking and cloth
- Vinegar — Acetobacter converts alcohol into acetic acid
- Dairy products — Lactobacillus converts the lactose of milk into lactic acid, which curdles the milk protein; this gives curd, and butter and cheese follow from it
- Silage — green fodder is packed into a pit, and bacterial fermentation preserves it as cattle feed for the dry season
The curd in every Indian kitchen is this chemistry in action. A spoonful of old curd added to warm milk introduces Lactobacillus; within hours the lactose has become lactic acid, the acid has set the protein, and the milk is curd — which is also why curd tastes sour and why it will not set if the milk is too cold for the bacteria to work.
Now notice what these processes have in common, because it is the point of the section. In each one the bacteria are allowed to break something down partly and then stopped — by drying the tea, by scraping the hide, by refrigerating the curd.
A controlled, incomplete decay is a useful process; the same decay allowed to run to completion is spoilage. So the whole industrial skill lies in knowing when to stop, and that is the honest connection between this section and the harmful effects in the next one.
What harm do bacteria do to food, crops and people?
Bacteria spoil food, cause diseases in plants and animals, and can be deliberately misused.
Spoilage of food. Saprophytic bacteria settle on food and decompose it, making it sour, slimy, discoloured and foul-smelling, and unfit to eat. Some go further and produce toxins that cause food poisoning — Clostridium botulinum growing in badly sealed canned food is the standard example, and its toxin is dangerous in very small amounts.
How spoilage is prevented, and every method works by denying the bacteria something they need:
- Refrigeration and freezing — cold slows their enzymes almost to a stop
- Boiling and pasteurisation — heat kills the bacteria present
- Canning — the food is sterilised and then sealed so that none can get in
- Drying — bacteria cannot grow without water, which is why dried fish, papad and dehydrated foods keep
- Salting and sugaring — a strong salt or sugar solution draws water out of the bacteria by osmosis
- Pickling — the acid of vinegar or lemon, and a layer of oil, make conditions the bacteria cannot tolerate
- Chemical preservatives such as sodium benzoate
Diseases of plants. Citrus canker, blight of rice, crown gall, wilt of tomato and brinjal, ring rot of potato, and soft rot of vegetables. They can destroy a large part of a crop, and control depends on disease-free seed, resistant varieties, crop rotation, removing infected plants and spraying.
Diseases of animals and humans. Cholera, typhoid, tuberculosis, tetanus, diphtheria, whooping cough, pneumonia, leprosy and plague in humans; anthrax and brucellosis in cattle. These are the subject of a later chapter in this course.
Bio-weapons. Certain highly infectious bacteria, or their resistant spores, can be released deliberately to cause disease on a large scale. Anthrax spores are the usual example, because they survive for long periods, spread easily and are hard to detect. Such use is prohibited internationally, and the danger is that the material is cheap to produce compared with any other weapon of comparable effect.
Now put the harmful list beside the useful one, because the pairing is the real content of this chapter.
- The bacteria that spoil food are the same kind that decompose dead matter and return its nitrogen to the soil
- The bacteria that set curd are doing what the bacteria that sour milk do — and sour milk and curd differ mainly in whether we chose it
- The denitrifiers that impoverish a field are what keeps nitrogen in circulation
- The bacteria that cause disease are close relatives of the ones that give us antibiotics
So there is no separate set of harmful bacteria and useful bacteria. There is one set of organisms that break substances down and build nitrogen compounds, and the label depends on where they are working and whether we wanted it.
That is why every method of food preservation is about controlling conditions rather than destroying an enemy — and why a refrigerator, a jar of pickle and a bag of salt are three different ways of saying the same thing to the same organism.
Spoilage of food. Saprophytic bacteria settle on food and decompose it, making it sour, slimy, discoloured and foul-smelling, and unfit to eat. Some go further and produce toxins that cause food poisoning — Clostridium botulinum growing in badly sealed canned food is the standard example, and its toxin is dangerous in very small amounts.
How spoilage is prevented, and every method works by denying the bacteria something they need:
- Refrigeration and freezing — cold slows their enzymes almost to a stop
- Boiling and pasteurisation — heat kills the bacteria present
- Canning — the food is sterilised and then sealed so that none can get in
- Drying — bacteria cannot grow without water, which is why dried fish, papad and dehydrated foods keep
- Salting and sugaring — a strong salt or sugar solution draws water out of the bacteria by osmosis
- Pickling — the acid of vinegar or lemon, and a layer of oil, make conditions the bacteria cannot tolerate
- Chemical preservatives such as sodium benzoate
Diseases of plants. Citrus canker, blight of rice, crown gall, wilt of tomato and brinjal, ring rot of potato, and soft rot of vegetables. They can destroy a large part of a crop, and control depends on disease-free seed, resistant varieties, crop rotation, removing infected plants and spraying.
Diseases of animals and humans. Cholera, typhoid, tuberculosis, tetanus, diphtheria, whooping cough, pneumonia, leprosy and plague in humans; anthrax and brucellosis in cattle. These are the subject of a later chapter in this course.
Bio-weapons. Certain highly infectious bacteria, or their resistant spores, can be released deliberately to cause disease on a large scale. Anthrax spores are the usual example, because they survive for long periods, spread easily and are hard to detect. Such use is prohibited internationally, and the danger is that the material is cheap to produce compared with any other weapon of comparable effect.
Now put the harmful list beside the useful one, because the pairing is the real content of this chapter.
- The bacteria that spoil food are the same kind that decompose dead matter and return its nitrogen to the soil
- The bacteria that set curd are doing what the bacteria that sour milk do — and sour milk and curd differ mainly in whether we chose it
- The denitrifiers that impoverish a field are what keeps nitrogen in circulation
- The bacteria that cause disease are close relatives of the ones that give us antibiotics
So there is no separate set of harmful bacteria and useful bacteria. There is one set of organisms that break substances down and build nitrogen compounds, and the label depends on where they are working and whether we wanted it.
That is why every method of food preservation is about controlling conditions rather than destroying an enemy — and why a refrigerator, a jar of pickle and a bag of salt are three different ways of saying the same thing to the same organism.
Exam tip
Exam tip: name the bacterium, the process and the product
Define an antibiotic as a substance produced by a MICRO-ORGANISM that kills or inhibits other micro-organisms.
Name bacterial antibiotics: streptomycin (from Streptomyces), tetracycline, chloramphenicol, erythromycin. Penicillin is from a FUNGUS — do not list it here.
Serum versus vaccine is the highest-value comparison in this lesson. A serum gives ready-made antibodies — immediate but short-lived, used for treatment. A vaccine contains killed, weakened or toxoid material and makes the body produce its own antibodies — slower but long-lasting, used for prevention.
Learn the four nitrogen groups with their genera. Nitrogen-fixing — Rhizobium in root nodules of legumes, and free-living Azotobacter and Clostridium. Ammonifying — dead matter to ammonia. Nitrifying — Nitrosomonas ammonia to nitrite, Nitrobacter nitrite to nitrate. Denitrifying — Pseudomonas, nitrate back to free nitrogen.
Get the two-step nitrification right — ammonia, then nitrite, then nitrate, with a different genus for each step.
Plants absorb nitrogen as NITRATE, never as nitrogen gas.
Denitrifying bacteria reduce soil fertility but are essential to the cycle — say both halves.
**Explain crop rotation through Rhizobium — a leguminous crop restores soil nitrogen free of cost.
For tea, say the bacteria develop the flavour, aroma and colour after the leaves are rolled. For leather, say they loosen the hair and fat so it can be scraped off, and that this stage is called curing.
Name Lactobacillus and lactic acid** for curd, and Acetobacter for vinegar.
And for preservation, give the method with the reason — drying removes water, salting draws it out by osmosis, pickling supplies acid, cold slows the enzymes.
Name bacterial antibiotics: streptomycin (from Streptomyces), tetracycline, chloramphenicol, erythromycin. Penicillin is from a FUNGUS — do not list it here.
Serum versus vaccine is the highest-value comparison in this lesson. A serum gives ready-made antibodies — immediate but short-lived, used for treatment. A vaccine contains killed, weakened or toxoid material and makes the body produce its own antibodies — slower but long-lasting, used for prevention.
Learn the four nitrogen groups with their genera. Nitrogen-fixing — Rhizobium in root nodules of legumes, and free-living Azotobacter and Clostridium. Ammonifying — dead matter to ammonia. Nitrifying — Nitrosomonas ammonia to nitrite, Nitrobacter nitrite to nitrate. Denitrifying — Pseudomonas, nitrate back to free nitrogen.
Get the two-step nitrification right — ammonia, then nitrite, then nitrate, with a different genus for each step.
Plants absorb nitrogen as NITRATE, never as nitrogen gas.
Denitrifying bacteria reduce soil fertility but are essential to the cycle — say both halves.
**Explain crop rotation through Rhizobium — a leguminous crop restores soil nitrogen free of cost.
For tea, say the bacteria develop the flavour, aroma and colour after the leaves are rolled. For leather, say they loosen the hair and fat so it can be scraped off, and that this stage is called curing.
Name Lactobacillus and lactic acid** for curd, and Acetobacter for vinegar.
And for preservation, give the method with the reason — drying removes water, salting draws it out by osmosis, pickling supplies acid, cold slows the enzymes.
Did you know
Why a bag of salt and a refrigerator do the same job
Every method of keeping food from spoiling looks different, and every one of them is the same idea applied in a different place.
Bacteria need four things to multiply: water, warmth, food and conditions that are not too acidic or too salty. Take away any one and they stop.
A refrigerator removes the warmth. The bacteria are still there and still alive, and their enzymes have simply slowed almost to a halt — which is why food keeps for days rather than for ever, and why it spoils quickly once taken out.
Drying removes the water. A papad, a piece of dried fish or a handful of raisins has almost no free water in it, and a bacterium cannot carry out a single reaction without a watery medium. Dried food needs no cold at all.
Salting and sugaring remove the water in a cleverer way. The food is still moist, but the water is in a strong solution — so water is drawn out of the bacteria by osmosis, and they shrivel. That is why a jar of jam and a strip of salted fish both keep, and why the sugar and the salt have to be generous to work.
Pickling changes the conditions rather than the supply. The acid of vinegar or lemon juice, and the layer of oil on top, make an environment most bacteria cannot tolerate — and the oil also keeps out the air.
Boiling and canning take the direct route and kill what is present, then keep more from arriving.
So five methods that look nothing alike are five ways of denying a bacterium one requirement.
And that also explains the failures. A jar of pickle with too little oil grows mould on top. A packet of dried food left open in the monsoon takes up moisture and spoils. Milk boiled and then left standing sours anyway, because boiling killed what was in it and nothing stopped more arriving.
Preservation is not a war against bacteria but a negotiation over conditions — which is why the same Indian kitchen keeps pickles for a year on the shelf, curd for a day outside the fridge, and milk for a week inside it, all without anybody ever removing a single bacterium.
Bacteria need four things to multiply: water, warmth, food and conditions that are not too acidic or too salty. Take away any one and they stop.
A refrigerator removes the warmth. The bacteria are still there and still alive, and their enzymes have simply slowed almost to a halt — which is why food keeps for days rather than for ever, and why it spoils quickly once taken out.
Drying removes the water. A papad, a piece of dried fish or a handful of raisins has almost no free water in it, and a bacterium cannot carry out a single reaction without a watery medium. Dried food needs no cold at all.
Salting and sugaring remove the water in a cleverer way. The food is still moist, but the water is in a strong solution — so water is drawn out of the bacteria by osmosis, and they shrivel. That is why a jar of jam and a strip of salted fish both keep, and why the sugar and the salt have to be generous to work.
Pickling changes the conditions rather than the supply. The acid of vinegar or lemon juice, and the layer of oil on top, make an environment most bacteria cannot tolerate — and the oil also keeps out the air.
Boiling and canning take the direct route and kill what is present, then keep more from arriving.
So five methods that look nothing alike are five ways of denying a bacterium one requirement.
And that also explains the failures. A jar of pickle with too little oil grows mould on top. A packet of dried food left open in the monsoon takes up moisture and spoils. Milk boiled and then left standing sours anyway, because boiling killed what was in it and nothing stopped more arriving.
Preservation is not a war against bacteria but a negotiation over conditions — which is why the same Indian kitchen keeps pickles for a year on the shelf, curd for a day outside the fridge, and milk for a week inside it, all without anybody ever removing a single bacterium.
Exam relevance
Why does NEET keep returning to the nitrogen cycle?
Because the nitrogen cycle is examined in two Class 11 chapters and again in Class 12, and the genus names are asked directly.
This is the foundation for Class 11 Biology Biological Classification and Mineral Nutrition, and Class 12 Ecosystem and Microbes in Human Welfare, examined in NEET. Mineral Nutrition covers nitrogen metabolism in detail — biological nitrogen fixation, the nitrogenase enzyme, the structure and function of the root nodule, and the conversion of nitrate to ammonia inside the plant. The four groups of bacteria named here are the ones that chapter examines.
The genera are asked by name. Questions pair Rhizobium, Azotobacter, Nitrosomonas, Nitrobacter and Pseudomonas with the step each performs. **The two-stage nitrification — Nitrosomonas for ammonia to nitrite and Nitrobacter for nitrite to nitrate — is the commonest match-the-column item in the topic, and getting the order and the genus the right way round is exactly what it tests.
The root nodule becomes a symbiosis topic. Class 11 explains the mutual benefit, the leghaemoglobin that protects nitrogenase from oxygen, and why nitrogen fixation requires anaerobic conditions inside an aerobic plant. Assertion-reason questions on the root nodule are common, and the symbiotic relationship described here is the starting point.
Class 12 Microbes in Human Welfare covers almost every industrial use on this page.** Lactobacillus in curd, biofertilisers including Rhizobium and the cyanobacteria Nostoc and Anabaena, industrial fermentation products, and sewage treatment by microbes with the biochemical oxygen demand as its measure. It also covers antibiotics as microbial products, and the fungal origin of penicillin is examined there — so the boundary point made on this page matters for that chapter too.
Serum against vaccine becomes active and passive immunity. Class 12 Human Health and Disease names them active immunity — the body makes its own antibodies, slow to develop and long-lasting — and passive immunity — ready-made antibodies supplied, immediate and short-lived. Questions asking which type a stated example is are a recurring NEET item, and the antivenom and anti-tetanus serum examples used here are the ones that chapter uses.
Denitrification and the cycle appear in ecology. Class 12 Ecosystem covers nutrient cycling, the difference between a gaseous and a sedimentary cycle, and the role of decomposers. The point that the same decomposers spoil food and enrich soil is what that chapter formalises as the recycling of nutrients through the ecosystem.
Food preservation is an applied topic. Class 12 covers microbial spoilage and the principles of preservation, and the reasoning used here — remove water, warmth, or tolerable conditions — is what makes each method explicable rather than a list.
What the questions look like. For board work, expect name three antibiotics obtained from bacteria, distinguish serum from vaccine, explain the role of nitrogen-fixing, nitrifying and denitrifying bacteria, describe the part played by bacteria in curing tea and tanning leather, and give four harmful effects with examples. Every answer wants the genus named where there is one. For NEET, expect genus-to-step matching, root-nodule items, active-and-passive immunity, and biofertiliser questions.
How board and competitive emphasis differ. A board paper rewards the named bacterium with the process and the product. A competitive paper assumes all three and asks about nitrogenase, leghaemoglobin, or which immunity a given example represents.
The single trap that costs the most marks. Saying that plants absorb nitrogen from the air. They cannot use nitrogen gas at all, however much of it surrounds them — a root absorbs nitrogen only as nitrate, and the whole point of the four groups of bacteria is to get from one to the other and back. The defence is to trace the route in your answer: nitrogen gas, then fixed compounds, then ammonia, then nitrite, then nitrate, then into the root — because once that chain is written down, every genus has an obvious place in it and nothing can be put in the wrong step.
This is the foundation for Class 11 Biology Biological Classification and Mineral Nutrition, and Class 12 Ecosystem and Microbes in Human Welfare, examined in NEET. Mineral Nutrition covers nitrogen metabolism in detail — biological nitrogen fixation, the nitrogenase enzyme, the structure and function of the root nodule, and the conversion of nitrate to ammonia inside the plant. The four groups of bacteria named here are the ones that chapter examines.
The genera are asked by name. Questions pair Rhizobium, Azotobacter, Nitrosomonas, Nitrobacter and Pseudomonas with the step each performs. **The two-stage nitrification — Nitrosomonas for ammonia to nitrite and Nitrobacter for nitrite to nitrate — is the commonest match-the-column item in the topic, and getting the order and the genus the right way round is exactly what it tests.
The root nodule becomes a symbiosis topic. Class 11 explains the mutual benefit, the leghaemoglobin that protects nitrogenase from oxygen, and why nitrogen fixation requires anaerobic conditions inside an aerobic plant. Assertion-reason questions on the root nodule are common, and the symbiotic relationship described here is the starting point.
Class 12 Microbes in Human Welfare covers almost every industrial use on this page.** Lactobacillus in curd, biofertilisers including Rhizobium and the cyanobacteria Nostoc and Anabaena, industrial fermentation products, and sewage treatment by microbes with the biochemical oxygen demand as its measure. It also covers antibiotics as microbial products, and the fungal origin of penicillin is examined there — so the boundary point made on this page matters for that chapter too.
Serum against vaccine becomes active and passive immunity. Class 12 Human Health and Disease names them active immunity — the body makes its own antibodies, slow to develop and long-lasting — and passive immunity — ready-made antibodies supplied, immediate and short-lived. Questions asking which type a stated example is are a recurring NEET item, and the antivenom and anti-tetanus serum examples used here are the ones that chapter uses.
Denitrification and the cycle appear in ecology. Class 12 Ecosystem covers nutrient cycling, the difference between a gaseous and a sedimentary cycle, and the role of decomposers. The point that the same decomposers spoil food and enrich soil is what that chapter formalises as the recycling of nutrients through the ecosystem.
Food preservation is an applied topic. Class 12 covers microbial spoilage and the principles of preservation, and the reasoning used here — remove water, warmth, or tolerable conditions — is what makes each method explicable rather than a list.
What the questions look like. For board work, expect name three antibiotics obtained from bacteria, distinguish serum from vaccine, explain the role of nitrogen-fixing, nitrifying and denitrifying bacteria, describe the part played by bacteria in curing tea and tanning leather, and give four harmful effects with examples. Every answer wants the genus named where there is one. For NEET, expect genus-to-step matching, root-nodule items, active-and-passive immunity, and biofertiliser questions.
How board and competitive emphasis differ. A board paper rewards the named bacterium with the process and the product. A competitive paper assumes all three and asks about nitrogenase, leghaemoglobin, or which immunity a given example represents.
The single trap that costs the most marks. Saying that plants absorb nitrogen from the air. They cannot use nitrogen gas at all, however much of it surrounds them — a root absorbs nitrogen only as nitrate, and the whole point of the four groups of bacteria is to get from one to the other and back. The defence is to trace the route in your answer: nitrogen gas, then fixed compounds, then ammonia, then nitrite, then nitrate, then into the root — because once that chain is written down, every genus has an obvious place in it and nothing can be put in the wrong step.
Key takeaways
Bacteria in medicine, agriculture and industry: quick revision
- Antibiotic — a substance produced by a micro-organism that kills or inhibits others. From bacteria: streptomycin (Streptomyces), tetracycline, chloramphenicol, erythromycin, neomycin. Penicillin comes from a FUNGUS.
- Many antibiotics work by stopping the bacterium building its cell wall, or making protein, or copying its genetic material.
- Serum — ready-made antibodies from an injected animal, usually a horse. Immediate but short-lived; used for treatment after exposure. Anti-tetanus serum, anti-diphtheria serum, antivenom.
- Vaccine — killed, weakened or toxoid material that makes the body produce its own antibodies. Slow to develop but long-lasting; used for prevention. BCG, TAB, DPT.
- A serum is a loan; a vaccine is a lesson.
- Nitrogen is four-fifths of the air and no plant can use it directly — roots absorb it only as nitrate.
- Nitrogen-fixing bacteria: symbiotic Rhizobium in the root nodules of legumes (pea, gram, groundnut, beans, soya); free-living Azotobacter and Clostridium; plus the cyanobacteria Nostoc and Anabaena in paddy fields.
- Ammonifying (putrefying) bacteria convert the nitrogen of dead matter and excreta into ammonia.
- Nitrifying bacteria work in two steps: Nitrosomonas ammonia to nitrite, Nitrobacter nitrite to nitrate.
- Denitrifying bacteria — Pseudomonas, Thiobacillus — return free nitrogen to the air. They lower soil fertility but keep the cycle turning, and they thrive in waterlogged soil, which is why draining and ploughing help.
- Crop rotation with a legume restores soil nitrogen free of cost, because Rhizobium fixes nitrogen while the crop grows.
- Curing of tea — rolled leaves left warm and moist; bacteria and the leaf's enzymes develop the flavour, aroma and colour. The same for tobacco, coffee and cocoa.
- Tanning of leather — the soaked hide has its hair and fat loosened by bacteria so they can be scraped off; this stage is curing, and tannin follows.
- Other uses: retting of jute and flax to free the fibres; vinegar from Acetobacter; curd from Lactobacillus turning lactose into lactic acid; silage for cattle.
- Spoilage of food — saprophytic bacteria make food sour, slimy and foul; Clostridium botulinum produces a dangerous toxin in badly canned food.
- Preservation denies the bacteria something: cold slows the enzymes; boiling and canning kill and exclude; drying removes water; salting and sugaring draw water out by osmosis; pickling supplies acid and excludes air; preservatives are added.
- Plant diseases: citrus canker, blight of rice, crown gall, wilt of tomato, ring rot of potato.
- Animal and human diseases: cholera, typhoid, tuberculosis, tetanus, diphtheria, pneumonia; anthrax in cattle.
- Bio-weapons — highly infectious bacteria or their spores, such as anthrax spores, released deliberately; prohibited internationally.
- There is no separate set of useful and harmful bacteria — the same organisms decompose and fix nitrogen, and the label depends on where they work and whether we wanted it.
Name any bacterial process on this page and say which requirement a preservation method would remove to stop it — if the answer comes without hesitation, this chapter is secure.
- Many antibiotics work by stopping the bacterium building its cell wall, or making protein, or copying its genetic material.
- Serum — ready-made antibodies from an injected animal, usually a horse. Immediate but short-lived; used for treatment after exposure. Anti-tetanus serum, anti-diphtheria serum, antivenom.
- Vaccine — killed, weakened or toxoid material that makes the body produce its own antibodies. Slow to develop but long-lasting; used for prevention. BCG, TAB, DPT.
- A serum is a loan; a vaccine is a lesson.
- Nitrogen is four-fifths of the air and no plant can use it directly — roots absorb it only as nitrate.
- Nitrogen-fixing bacteria: symbiotic Rhizobium in the root nodules of legumes (pea, gram, groundnut, beans, soya); free-living Azotobacter and Clostridium; plus the cyanobacteria Nostoc and Anabaena in paddy fields.
- Ammonifying (putrefying) bacteria convert the nitrogen of dead matter and excreta into ammonia.
- Nitrifying bacteria work in two steps: Nitrosomonas ammonia to nitrite, Nitrobacter nitrite to nitrate.
- Denitrifying bacteria — Pseudomonas, Thiobacillus — return free nitrogen to the air. They lower soil fertility but keep the cycle turning, and they thrive in waterlogged soil, which is why draining and ploughing help.
- Crop rotation with a legume restores soil nitrogen free of cost, because Rhizobium fixes nitrogen while the crop grows.
- Curing of tea — rolled leaves left warm and moist; bacteria and the leaf's enzymes develop the flavour, aroma and colour. The same for tobacco, coffee and cocoa.
- Tanning of leather — the soaked hide has its hair and fat loosened by bacteria so they can be scraped off; this stage is curing, and tannin follows.
- Other uses: retting of jute and flax to free the fibres; vinegar from Acetobacter; curd from Lactobacillus turning lactose into lactic acid; silage for cattle.
- Spoilage of food — saprophytic bacteria make food sour, slimy and foul; Clostridium botulinum produces a dangerous toxin in badly canned food.
- Preservation denies the bacteria something: cold slows the enzymes; boiling and canning kill and exclude; drying removes water; salting and sugaring draw water out by osmosis; pickling supplies acid and excludes air; preservatives are added.
- Plant diseases: citrus canker, blight of rice, crown gall, wilt of tomato, ring rot of potato.
- Animal and human diseases: cholera, typhoid, tuberculosis, tetanus, diphtheria, pneumonia; anthrax in cattle.
- Bio-weapons — highly infectious bacteria or their spores, such as anthrax spores, released deliberately; prohibited internationally.
- There is no separate set of useful and harmful bacteria — the same organisms decompose and fix nitrogen, and the label depends on where they work and whether we wanted it.
Name any bacterial process on this page and say which requirement a preservation method would remove to stop it — if the answer comes without hesitation, this chapter is secure.